1. Field of the Invention
The present invention relates to a power supply frequency regulating device for a motor utilizing ultrasonic vibration.
2. Related Backround Art
Prior art of such device is for example disclosed in the U.S. Pat. No. 4,510,411 which describes a technology for controlling the exciting frequency of an AC power supply at the resonance frequency of the motor.
Also the U.S. Pat. Application Ser. No. 929,351 filed Nov. 12, 1986, now Pat. No. 4,743,788, issued May 10, 1988 and assigned to the assignee of the present application discloses a technology for controlling the exciting frequency at a frequency range higher than the resonance frequency of the motor.
In this prior art the exciting frequency is controlled on the assumption that the amplitude of vibration of the vibration wave driven motor is correlated with the exciting frequency, or that a fixed amplitude can be obtained for a frequency as long as external conditions remain unchanged.
However the present inventor has subsequently found, experimentally, that the frequency-amplitude characteristic of a vibration wave driven motor shows a strong hysteresis. Therefore, in the course of control of the power supply frequency to an optimum vlaue, such optimizing operation becomes impossible if the function point on the frequency-amplitude characteristic curve moves to the undesired side of the hysteresis in the motor drive.